Two Auckland beaches have been soiled with sewage after torrential rain earlier today caused flash floods and chaos on the North Shore.

A wastewater overflow was detected by Auckland Council after the heavy downpour.

Two swim alerts - in Castor Bay and Milford on the North Shore - came into effect today due to a "high risk" water quality.

Swimmers were advised to avoid the water.

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Castor Bay and Milford beaches have been contaminated with sewage. Image / Swimsafe
Castor Bay and Milford beaches have been contaminated with sewage. Image / Swimsafe

Watercare operations controller wastewater Sifa Pole said the deluge of stormwater engulfed the wastewater system and a wet weather overflow occurred, into the local waterway, which flows out to sea.

"These engineered overflows are designed to prevent wastewater from backing up and overflowing on private property.

"The sensors at pump stations in Castor Bay and Milford operated as they are designed to do- and went off when the heavy rainstorm occurred which caused substantial flooding across parts of the North Shore. The sensors also automatically alerted the Safeswim programme."

Pole explained that stormwater finds its way into the wastewater system via illegal or non-compliant private drainage, as well as older or leaky pipes (often damaged by tree roots). During flooding, stormwater can also enter wastewater pipes via private gully traps. The stormwater from a single house can displace the equivalent wastewater from more than 40 households

"Watercare is developing a comprehensive inflow and infiltration programme to help reduce the amount of stormwater entering the network," Pole said.

Just over 62mm of rain fell between 8am and 9am this morning - the wettest hour in at least eight years - as the slow-moving storm cell delivered a deluge over the northern suburb of Rosedale.

Two motorists needed rescuing from their vehicles after they became submerged in the waist-deep floodwater.

About 14 homes were affected by flooding and firefighters cleared blocked drains to help the water drain away.

This comes just weeks after Takapuna beach was temporarily declared unsafe for swimming after stormwater ran into the sea.

Lifeguards shut down the beach at 3.30pm on January 1, when a fault in the network caused a stormwater drain to overflow.

The flags were taken down and the beach was closed for swimming for the remainder of the day - with the water quality labelled "high risk".